Exodus 15

 As the Israelites left the Red Sea - 

"...they went three days in the wilderness, and found no water.

23 ¶ And when they came to Marah, they could not drink of the waters of Marah, for they were bitter: therefore the name of it was called Marah.

24 And the people murmured against Moses, saying, What shall we drink?

25 And he cried unto the Lord; and the Lord shewed him a tree, which when he had cast into the waters, the waters were made sweet: there he made for them a statute and an ordinance, and there he proved them..."  (Exodus 15:22–25)

In my mind, bitter water may have been water that had metallic, sulfuric, or terrible taste.  In my mind, a tree that could sweeten the taste in itself is a miracle.